如何根据键值推断类型？

Question

我正在尝试推断

getAllRaces()

的类型是

() => Race[]

.

到目前为止我试过：

type CollectionMap = {
    races: Race[]
    horses: Horse[]
}

type Race = {
    date: Date
}

type Horse = {
    name: string
}

type UnionizeKeys<T> = {
    [k in keyof T]: k
}[keyof T]

type CollectionName = UnionizeKeys<CollectionMap> // "races" | "horses"

// Failed attempts

const getAll1 = (name: CollectionName) =>  [] as CollectionMap[name];
// 💥 Type 'name' cannot be used as an index type.(2538)

const getAll2 = (name: CollectionName) =>  [] as CollectionMap[typeof name as const];
// 💥 A 'const' assertions can only be applied to references to enum members,
//    or string, number, boolean, array, or object literals.(1355)

const getAll = (name: CollectionName) =>  [] as CollectionMap[typeof name];

const getAllRaces = () => getAll('races')
// ❌ const getAllRaces: () => Race[] | Horse[]
// ✅ const getAllRaces: () => Race[]

TypeScript 游乐场

非常感谢您的帮助

Answer 1

如果你想让

getAll()

的返回类型依赖于

name

输入，你可以在K中让它成为

generic

，

name

输入的类型，其中

K

是constrained到

keyof CollectionMap

（和你的

CollectionName

类型一样，写得比回旋处更直接

UnionizeKeys

）：

const getAll = <K extends keyof CollectionMap>(
    name: K
): CollectionMap[K] => [];

返回类型只是 索引访问类型

CollectionMap[K]

，

CollectionMap

的属性类型，键类型为

K

.

让我们测试一下：

const getAllRaces = () => getAll('races')
// const getAllRaces: () => Race[]
const getAllHorses = () => getAll('horses');
// const getAllHorses: () => Horse[]

看起来不错。

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